C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4

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Electrochemically reduced graphene oxide sheets for usein high performance supercapacitors

Jiang Yang, Sundaram Gunasekaran *

Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706, USA

A R T I C L E I N F O

Article history:

Received 17 April 2012

Accepted 1 August 2012

Available online 9 August 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.08.003

* Corresponding author.E-mail address: guna@wisc.edu (S. Gunas

A B S T R A C T

Graphene oxide (GO) was reduced by a rapid, effective and eco-friendly electrochemical

method of repetitive cathodic cyclic potential cycling, without using any reducing reagents.

The electrochemically reduced graphene oxide (ERGO) was characterized by UV–vis, EIS

and zeta-potential measurements. Most of the oxygen functional groups in ERGO were suc-

cessfully removed resulting in smaller charge transfer resistance. However, some electro-

chemically stable residuals still remained, enabling ERGO to facilitate electrolyte

penetration and pseudocapacitance. Since ERGO was readily stabilized by cathodic poten-

tial cycling, it exhibited an outstanding stability in cycle life, nearly with no capacitive loss

from the second cycle on. A specific capacitance of 223.6 F g�1 was achieved at 5 mV s�1,

which makes the ERGO a competitive material for electrochemical energy storage.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Supercapacitors, also known as electric double-layer capaci-

tors (EDLCs) or ultracapacitors, have attracted much attention

from the research community in recent years [1]. These

devices can serve as power sources by themselves or in com-

bination with other devices and have many appealing proper-

ties such as rapid charging/discharging rate, excellent energy

density, high power density and long lifetime, possibly over

one million cycles [2]. The energy density of supercapacitors

are several orders of magnitude higher than that of conven-

tional dielectric capacitors [3], because energy density is in-

versely related to the double layer thickness. There are two

mechanisms of energy storage in supercapacitors: (1) EDLCs

where capacitance is aroused by reversible electrolyte ion

adsorption at electrode-electrolyte interface and formation

of a double layer with high-surface-area carbon materials as

typical examples and (2) redox capacitors where battery-type

redox reactions generate pseudocapacitance with certain me-

tal oxides or conducting polymers as electrode materials.

There are numerous studies on the development of superca-

er Ltd. All rights reservedekaran).

pacitive materials of carbon [3–5], metal oxides [6,7], conduct-

ing polymers [8] and composites [9]. Though energy densities

based on pseudocapacitance are much higher than that of

EDLC, the faradic reactions within pseudocapacitive elec-

trodes can cause phase changes, limiting their lifetime and

power densities [10]. Meanwhile, practical applications of

pseudocapacitive materials are also confined to their low

electrical conductivity, poor compatibility with organic elec-

trolytes [1] and relatively high cost. Therefore, it is essential

to look for new, inexpensive, and environmentally friendly

energy storage systems.

Carbon materials are known to be the most ideal as sup-

ercapacitors to provide DL capacitance [11], with their capaci-

tance directly proportional to surface area and porosity.

Different carbon-based materials for high power and energy

supercapacitor applications have been extensively reviewed

[12,13], including activated carbon, carbon nanotubes, carbon

aerogels, graphene and carbon-based composites. Although

carbon nanotubes exhibit large surface area, high electrical

conductivity along the tubes and good capacitive performance

[14], the high cost hinders their commercialization [1]. Graph-

.

C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4 37

ene is a single-atom-thick planar sheet of sp2-bonded carbon

nanostructure with a large theoretical specific surface area

of 2630 m2 g�1, high intrinsic in-plane electrical conductivity,

and robust mechanical strength [1,10,15]. Graphene materials

obtained from GO now can be readily produced and sold by

tons at low cost [16]. More importantly, instead of relying on

pore distribution in activated carbon (AC), the electronic and

capacitance properties of graphene are layer-dependent [17].

Many approaches have been developed to produce graphene

physically and chemically such as micromechanical [18] or

electrochemical [19] exfoliation, thermally growing from sili-

con carbide [20], and chemical vapor deposition [21]. Of these,

solution-based approach is regarded as the most promising for

mass production. Exfoliated GO can be reduced chemically

[5,11,22], thermally [23], electrochemically [15,24–26]or by

microwave-assisted techniques [1,27]. Considering most

reducing reagents such as hydrazine, alkaline, ethylenedia-

mine, NaBH4 and urea are toxic, corrosive or even explosive,

there are serious safety and environmental issues. Addition-

ally, possible stabilizers involved in these processes to im-

prove dispersion of reduced GO may be sometimes

undesirable due to their degradation on electronic properties

[5]. Therefore a greener electrochemical route with the possi-

bility to mass produce is highly desirable. Recently, superca-

pacitors based on ERGO in aqueous electrolytes using

cathodic constant-potential reduction were reported [25,26].

The cycling durability may not be as good as cyclic voltamme-

try (CV)-reduced GO if without further reduction or manipula-

tion, as capacitance decrease in first multiple cycles were

observed [26]. This also widely exists in GO reduced only with

one reducing agent [3,5], due to incomplete removal of electro-

chemically unstable oxygen species.

Herein we report using ERGO, which is reduced from GO by

repetitive cathodic cyclic potential cycling, for supercapacitor

application in aqueous 1.0 M H2SO4 electrolyte. The high nega-

tive potential (complete reduced potential: �1.5 V) allowed

overcoming energy barriers for oxygen functionalities [24],

resulting in the elimination of electrochemically unstable

groups. Remaining oxygen groups played a significant role in

extra pseudocapacitive effects. During the reduction process,

ERGO was readily stabilized electrochemically with barely

any capacitive loss for up to 1000 galvanometric cycles. The

cyclic voltammetry and galvanostatic charge–discharge results

show that ERGO has high gravimetric and volumetric capaci-

tance with satisfactory rate capability and cycling stability.

2. Experimental

2.1. Synthesis of ERGO

GO was synthesized from graphite powder using a modified

Hummer’s method, and then reduced by intensive repetitive

CV cycling as we previously reported [15]. Briefly, 3.0 g graph-

ite powder, 2.5 g K2S2O8 and 2.5 g P2O5 were mixed with 12 mL

98% H2SO4 solution at 80 �C for 4.5 h to obtain pre-oxidized

graphite. The mixture was then diluted with 0.5 L water and

kept at 80 �C for 12 h. The resulting solution was filtered,

washed and vacuum-dried at room temperature overnight.

The products were put into 120 mL 98% H2SO4 with ice water

bath and stirred, followed by gradual addition of 15 g KMnO4

with continuous stirring at 40 �C for 0.5 h and 90 �C for 1.5 h.

Further, 250 mL water was added with stirring at 105 �C for

25 min and additionally at room temperature for 2 h. Finally,

0.7 L water and 20 mL 30% (w/w) H2O2 were added to termi-

nate the reaction. The as-obtained products were filtered,

washed with 3 M HCl solution to remove metal ions, and

repeatedly washed with water until neutral pH. It was further

purified by dialysis for one week to remove impurities and re-

suspended by ultrasonication in water to obtain a homoge-

neous GO solution. Stable exfoliated GO dispersion was

achieved by the negative electrostatic repulsion from ioniza-

tion of carboxyl and phenolic hydroxyl groups of GO and

therefore made it much easier to be used for electrode coating

and modification compared to comparatively insoluble re-

duced GO. Glassy carbon electrode (GCE, 3 mm in diameter)

was used as the conductive substrate. It was carefully pol-

ished on a fine microcloth successively with 0.3 and 0.05 lm

alumina slurry (Beuhler) until a mirror-shine surface was ob-

tained, and then rinsed with water. A sonication step was per-

formed consecutively in ethanol and water, and the GCE was

then dried at room temperature. Three milliliters GO solution

of 1 mg mL�1 concentration was drop coated on a conductive

electrode surface (e.g. GCE, Au) and dried under ambient con-

ditions. GO was electrochemically reduced by CV scanning

from 0.0 to �1.5 V in N2-purged 0.05 M pH 5.0 PBS (Na2HPO4/

NaH2PO4) for 100 cycles, and then rinsed with water and dried

at room temperature. Large-scale electrochemical reduction

of GO was performed using the same repetitive cyclic poten-

tial sweeping in 3-electrode configuration under the same

conditions for longer period of time sealed to minimize evap-

orations. Doubly-distilled Milli-Q water (18.2 MX) was used

throughout the study.

2.2. Instrumentation

Hitachi S-4800 (Hitachi, Japan) scanning electron microscope

(SEM) was employed for characterizing surface morphology

and UV–vis spectra were recorded with Cary 50 Bio UV–vis

spectrophotometer (Varian, CA, USA). Zeta potentials were

measured using 90Plus particle size analyzer with BI-Zeta op-

tion (Brookhaven Instruments Corporation, TX, USA). All elec-

trochemical tests including CV, galvanostatic charge/

discharge and EIS were carried out using CHI 660D electro-

chemical analyzer (CH Instruments, TX, USA) at ambient

temperature (25 ± 1 �C) in a three-electrode configuration,

with Pt wire counter electrode (CE), saturated Ag/AgCl (3 M

KCl) reference electrode (RE) and ERGO as the working elec-

trode (WE). 1 M H2SO4 was used as electrolytes and electro-

chemical impedance spectroscopy (EIS) was conducted from

0.1 Hz to 100 kHz with 5 mV AC perturbation in 1.0 M KCl con-

taining equimolar K4[Fe(CN)6]/K3[Fe(CN)6] redox probes at

open circuit potential.

3. Results and discussion

3.1. Characterizations of ERGO

The surface morphology of ERGO characterized by SEM is

shown in Fig. 1A. A few layers of GO sheets appear partially

packed with their basal planes, intensely crumpled and

Fig. 1 – (A) SEM image of ERGO. (B) UV–vis spectra of

20 lg mL�1 (a) GO and (b) ERGO. Upper right photographs

shows the appearance of GO and ERGO solutions while

lower left is the magnification of ERGO solution. (C) Zeta

potentials of ( ) GO and (j) ERGO under different pH.

38 C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4

folded into a typical wrinkled structure after electrochemical

reduction, similar to that of reduced GO sheets derived chem-

ically [22], thermally [23] or with microwave [27]. These geo-

metric wrinkling and rippling are caused by nanoscale

interlocking of GO sheets, providing enhanced mechanical

properties, reduced surface energy, increased surface rough-

ness and area [28]. Corrugated ERGO sheets are arranged

intermittently edge-to-edge with less wrinkled or flat sheets,

forming an interconnected film parallel to the substrate. We

previously reported that ERGO is composed of multilayers of

graphene sheets overlapped in parallel for interpenetration

[15]. Its properties possibly allow the design of thin-film sup-

ercapacitor devices. The corrugations in graphene sheets can

cause electron transfer rates 10-fold faster than at the basal

plane of graphite [29].

The change in GO after electrochemical reduction was

monitored by UV–vis spectroscopy (Fig. 1B). An obvious

absorption peak at 230 nm was observed for GO, correspond-

ing to the p! p* transition of aromatic C–C bonds [5]. After

electrochemical reduction, this absorption peak has red-

shifted to 270 nm in decreased intensity (as the arrow indi-

cated), indicating restored p conjugation within ERGO sheets

[30]. The spectra are close to those from hydrazine reduction

[22]. When dispersed in aqueous solution, GO showed a

brown color with good dispersion and stability which can last

for months, due to the abundant oxygenated groups on the

basal plane and edge of GO surface (Fig. 1B upper right inset).

In contrast, ERGO exhibited a slightly black color in the solu-

tion and many insoluble small particles are visually observa-

ble. The increase in hydrophobicity suggested the successful

elimination of electrochemically unstable oxygen-containing

functional groups after reduction.

To further explore the surface properties, zeta potentials

were measured as a function of pH (Fig. 1C). In the pH ranging

from 5 to 8, the zeta potential values of GO sheets decreased

from �29.0 to �37.3 mV and GO remained highly negatively

charged all through, which can be attributed to the presence

of oxygen species on the surface. The increase of negative

charge in zeta potential at pH 8 is caused by deprotonation

of carboxyl groups into carboxylate groups. From a general

colloidal science perspective, zeta potentials with absolute

values larger than 30 mV can result in a stable dispersion

[31]. Therefore, the GO dispersion can be well maintained

due to the electrostatic repulsion as observed in Fig. 1B. On

the other hand, ERGO showed zeta potential values around

zero. The small positive zeta potential values of ERGO at

pH < 7 are ascribed to adsorption of positively charged ions

(e.g. H+, Na+) in the aqueous solution, as a result of removal

of negatively charged oxygenated species, also observed in

thermally and chemically reduced GO [32]. These results are

indicative of efficient electrochemical reduction of GO.

3.2. Electrochemical characterizations of ERGO

As the typical method for evaluating the electron transfer

properties in carbon materials, CV analysis of redox reactions

of Fe(CN)63�/Fe(CN)6

4� was used to investigate the intrinsic

electrochemical behaviors of ERGO electrode (Fig. 2A). At the

same scan rate of 100 mV s�1, ERGO electrode showed a much

smaller peak-to-peak potential separation (DEp) and a larger

peak current than GO electrode, thus offering a faster electron

transfer rate and larger effective surface area at ERGO. The

well-defined redox peak pair demonstrates a favorable direct

electron transfer between ERGO electrode and redox species

[33]. As the scan rate increased, the anodic peak shifted

positively while the cathodic peak shifted negatively, generat-

Fig. 2 – (A) Cyclic voltammograms of (a) ERGO (b) GO and in 5.0 mM K3[Fe(CN)6] containing 1.0 M KCl at scan rate of 100 mV s�1.

Left figure shows ERGO at different scan rates of 10, 20, 30, 50, 80, 100, 200 and 300 mV s�1, with the arrow indicating

increasing scan rates. (B) Plot of peak currents vs. square root of scan rates. (C) Plot of anodic and cathodic peak potentials (a

and b), potential separation (c), and midpoint potential (d, nearly independent) vs. square root of scan rates. Inset shows w vs.

reciprocal of square root of scan rate.

C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4 39

Fig. 3 – Nyquist plots of (a) ERGO, (b) GO, (c) bare, (d) ERGO

after 1000 charge/discharge cycles in 1.0 M KCl containing

0.01 M K4[Fe(CN)6]/K3[Fe(CN)6]. Lower right inset: enlarged

high-frequency region. Upper left inset: equivalent circuit

[Rs(CdlRct)]CPE fitting ERGO electrode. Rs: solution resistance,

Rct: electron transfer resistance, Cdl: double layer

capacitance, CPE: constant phase element.

40 C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4

ing slightly larger DEp (from 70 to 103 mV) with increasing

scan rates which indicated quasi-reversible kinetics at ERGO

surface likely as a result of graphite basal plane structures

and edge-plane defects [34]. Meanwhile, a linear dependence

can be established between the peak currents and square root

of scan rates from 0.01 to 0.30 V s�1 (R = 0.9985 and 0.9972 and

linear Randles’ slope of 5.89 · 10�4 and �6.15 · 10�4 for anodic

and cathodic peaks, respectively), suggesting a diffusion-

controlled non-surface process (Fig. 2B). On the basis of this

linear dependency in the one-electron process and Randles-

Sevcik equation (Eq. (1)) for quasi-reversible processes [35],

the effective surface area (A) of ERGO electrode was calculated

as 0.106 ± 0.003 cm2.

Ip ¼ ð2:65� 105Þn3=2AD1=2C�m1=2 ð1Þ

where, D is diffusion coefficient (cm2 s�1), C* is concentration

of probe molecule (5 · 10�6 mol cm�3), n is the number of elec-

trons (=1). The result is apparently larger than the geometric

area of the electrode (0.071 cm2) by providing additional elec-

troactive sites and more effective contacts with redox species.

Using Nicholson’s method [33,36] in which DEp is a function of

the dimensionless kinetic parameter w, a further linear rela-

tion (R = 0.984) can be found for w vs. m1/2 (Fig. 2C inset), related

by k0 as Eq. (2):

w ¼ k0 D0

DR

� �a=2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiRT=ðpnFDÞ

pm�1=2 ð2Þ

where, a is the electron transfer coefficient (assumed as 0.5), D0

is the diffusion coefficient for K3[Fe(CN)6] (0.76 · 10�5 cm2 s�1),

DR is the diffusion coefficient for K4[Fe(CN)6], R, T (298 K), p and

F have their usual meanings. The standard heterogeneous rate

constant k0 was then estimated as 6.04 · 10�3 cm s�1 from the

slope of the above linear fit. This value is close to or higher than

that of ferricyanide for multilayer graphite [37], monolayer of

mechanically exfoliated graphene [38], bare and modified Au

electrodes [39], and commercial screen-printed graphite elec-

trodes [35]. Recent studies have suggested that graphene

might have more favorable electron transfer kinetics over

graphite [40]. Moreover, linear relationships of both Epa and Ep-

c(and DEp) on m1/2 (Fig. 2C) were found with close absolute val-

ues of slopes, and thus the formal potential or midpoint

potential ðEpaþEpc

2 Þ is independent of scan rates suggesting char-

acteristics of the redox reaction and good association of ERGO

with electrode surface in the solution conditions. Considering

w vs. m1/2 was also linear, a linear relationship of DEp on w�1 can

be built consistent with the model of alternative Nicholson’s

working curve developed by Paul et al. for quasi-reversible pro-

cess [41].

The electron transfer characteristics between electrolyte

and electrode interfacial surface were studied using EIS in

0.01 M [Fe(CN)6]3�/4� as shown by the Nyquist complex-plane

plots (Fig. 3). The semicircular part in the high frequency

region (Fig. 3 inset) represents electron-transfer-limiting pro-

cess with its effective diameter equal to the faradic charge

transfer resistance (Rct) which is responsible to the electron

transfer kinetics of redox reactions at the electrode-electrolyte

interface. It is obvious that GO exhibited a much larger Rct than

bare electrode, due to the presence of excessive oxygenated

species which arouse insulating characteristics and hinder

its electrochemical properties [11]. ERGO, on the other hand,

displayed a significantly reduced Rct (29.7 X) than GO, suggest-

ing enhanced electronic properties and successful elimination

of electrochemically unstable oxygen species during reduc-

tion, which can cause pseudocapacitance [11]. From the inter-

cept at real part equal to internal resistance (Ri), ERGO has the

smallest value indicating ERGO can achieve higher charge/dis-

charge rate and the removed oxygen species in GO mainly

contribute to the increase of Rct rather than Ri. Furthermore,

the straight line in the low frequency region, corresponding

to a diffusion-limiting process, is less inclined for ERGO, sug-

gesting more closely ideal capacitor behaviors [10] possibly

by forming more uniform diffusion paths for protons within

ERGO after reduction. GO electrode, however, displayed a par-

ticularity with a second semicircle in the low frequency region

which is the typical characteristic of diffusion through a finite

length diffusion layer. Hereby, the non-ideal line of ERGO can

be expressed by the serial constant phase element (CPE) as in

the equivalent circuit with the parameter CPE-P = 0.50 identi-

cal to a Warburg element, demonstrating ideal Cdl only

accounts for part of the capacitive performance. After 1000

charge/discharge cycles, Rct of ERGO increased to 52.5 X owing

to fewer hopping sites of oxygenated groups on ERGO in slow

kinetics faradic reactions after long cycling period [42], similar

to chemically reduced RGO behaviors [11]. The solution resis-

tance (Rs) only varied a little (2.9%) after 1000 cycles due to its

relative insensitivity to electrode surface.

3.3. Capacitive properties of ERGO

The capacitive behavior of ERGO was evaluated with 3-elec-

trode configuration in aqueous system at 100 mV s�1 (Fig. 4A).

Both GO and GC electrodes presented negligible current

responses, whereas a remarkably large current was seen in

0 100 200 300 400 500100

150

200

250

0

20

40

60

80

100(C)

Ret

entio

n / %

(B)

(A)

Fig. 4 – (A) Cyclic voltammograms of (a) ERGO (b) GC and (c) GO electrodes. Scan rate: 100 mV s�1. (B) Cyclic voltammograms of

ERGO at different scan rate from 10 to 500 mV s�1 (from j to a). (C) Capacitance retention ratio and specific capacitance as a

function of scan rate from 5 to 500 mV s�1. Electrolyte: 1.0 M H2SO4.

C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4 41

Fig. 5 – (A) Galvanostatic charge/discharge curves of ERGO-

based supercapacitor in 1.0 M H2SO4 at different current

densities of (a) 4, (b) 2, (c) 1, (d) 0.5 and (e) 0.2 mA cm�2. (B)

Dependence of capacitance of ERGO supercapacitor on

current density. Potential range: 0–0.8 V.

42 C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4

the CV profile of ERGO, which showed a typical rectangular

shape indicating an excellent capacitive behavior. At the same

time, a pair of redox peaks were found, attributed to quinone/

hydroquinone groups typically in carbon materials [43]. This

confirmed the residual electrochemically stable oxygen groups

are still actively involved in faradic redox reactions and con-

tribute to pseudocapacitance of ERGO, though the reduction

of electrochemically unstable oxygen groups greatly increased

capacitance. Additionally, the unremoved oxygen groups can

also increase hydrophilicity of ERGO to some degree (we have

reported a water contact angle of 72� [15]), allowing penetration

of aqueous electrolyte [11].

CV curves of ERGO electrodes were obtained at scan rates

from 5 to 500 mV s�1 (Fig. 4B). The current response increased

with scan rates and similar rectangular shape with a pair of

redox peaks is observed at all scan rates, with no obvious dis-

tortion (even at a scan rate of 10 V s�1, no obvious distortion is

observed). This superior acceptable capacitive performance is

due to the partial restoration of p-conjugation structure and

improved electronic conductivity. The specific capacitance

(Cm) of ERGO from CV can be calculated from Eq. (3):

Cm ¼jQf j þ jQbj

2mDVð3Þ

where, Qf and Qb are voltammetric charge integrated from area

of forward and backward scans under CV curves, respectively;

m is the mass of electrode materials; DV is the potential win-

dow. The capacitance and retention ratio (of Cm at 5 mV s�1)

of ERGO as a function of scan rate are shown in Fig. 4C. At

5 mV s�1, a specific capacitance of 223.6 F g�1 was obtained

and this value decreased with increasing scan rate, first dra-

matically and then gradually until a relatively stable value

(�170 F g�1) was reached, showing great rate capability. The ra-

pid decrease is because some oxygen groups that are still

remaining on ERGO are unable to participate in faradic redox

reactions at higher scan rates, leading to lower pseudocapaci-

tance [5]. This phenomenon can also be seen in Fig. 4B from

decreasing redox peak area with increasing scan rate. At

500 mV s�1, 75.6% of Cm can be maintained, while 178.4 F g�1

(80.0%) can be achieved at 100 mV s�1. This capacitance perfor-

mance is a better than those of GO reduced by hydrobromic

acid [11], GO reduced by microwave-solvothermal route with

hydrazine [44], hydrogen-induced exfoliated graphene [45],

and inkjet-printed and thermally reduced GO in the same elec-

trolyte [4]. Compared with other carbon materials, the specific

capacitance is higher than nanodiamond and onion-like-car-

bon (OLC) in the range of 20–40 F g�1 [46], and the low capaci-

tance of OLC is said to be caused by its hydrophobic surface

making it difficult to be dispersed in aqueous solution and/or

to be accessed by aqueous electrolyte. It is also higher than

mesoporous carbon synthesized from silica templates ranging

from 114 to 147 F g�1 [47] and is comparable to nanoporous car-

bon activated of �240 F g�1 at temperature above 950 �C [48].

Galvanostatic charge/discharge performance of ERGO elec-

trode was tested at different current densities (Fig. 5A). Sym-

metric charge/discharge curves of almost linear potential

variation in good triangle shape revealed a good capacitive per-

formance from electrical double layer capacitance and pseud-

ocapacitance. The specific capacitance here is calculated

depending on Eq. (4):

Cm ¼IDt

DVmð4Þ

where, I is the discharged current, Dt is the discharging time

and other parameters have identical meanings as before.

The highest Cm of 180.8 F g�1 at 0.2 mA cm�2 is ascribed to

the remaining electrochemically stable oxygen species in

ERGO generating extra pseudocapacitance and facilitated

electrolyte penetration [11]. Cm gradually decreased with in-

crease of discharging current densities, and stabilized around

150 F g�1 (Fig. 5B). This decrease, though small (�2.9% loss in

capacitance from 0.5 to 4 mA cm�2), is generally caused by

internal resistance drop (IR drop). The IR drop was observed

to be 3.5 mV at 0.2 mA cm�2 resulting in the low capacitance

loss.

The cycle life of the supercapacitor as an important factor for

practical applications[3] was examined at current density of

0.2 mA cm�2 for up to 1000 charge/discharge cycles (Fig. 6A).

The specific capacitance of ERGO electrode was 207.8 F g�1 in

the first cycle and remained almost constant around 180 F g�1

Fig. 6 – (A) Cycle life of ERGO electrode at the current density

of 0.2 mA cm�2 between 0 and 0.8 V. (B) Cyclic

voltammograms in 1.0 M H2SO4 at 100 mV s�1 of (a) ERGO

electrode, (b) after 1000 charge/discharge cycles and (c) after

storage for 1 week.

C A R B O N 5 1 ( 2 0 1 3 ) 3 6 – 4 4 43

from the second cycle on, revealing extraordinary capacitive

stability and high level of reversibility in repetitive charge/dis-

charge cycling. This strong stability is because during the elec-

trochemical reduction, ERGO has already been stabilized with

those electrochemically unstable deteriorating oxygen species

removed. Given the fact that GO with high oxygen contents

does not have a comparable capacitance (Fig. 4A), it can be

inferred that the residual covalently bonded oxygen species in

ERGO are reversible, stable and desirable, playing an important

part in its excellent performance. The CV curves of ERGO elec-

trode after 1000 cycles and one week storage (with a tight plastic

cap kept from air) were also compared to estimate the retention

ability (Fig. 6B). Virtually very little loss in capacitance can be

seen and therefore ERGO is suitable for high-performance sup-

ercapacitor applications.

4. Summary

We have demonstrated the supercapacitive application of

ERGO obtained by reducing GO by an easy, cost-effective and

eco-friendly electrochemical method without using any reduc-

ing agents. After reduction, p-conjugated structure in graph-

ene was repaired and electrochemically unstable oxygen

groups were rapidly removed, yielding greatly improved elec-

tron transfer kinetics and capacitive properties of ERGO. Mean-

while, residual oxygen groups on ERGO are considerably stable

and reversible in capacitive performance providing additional

faradic pseudocapacitance without damaging its electronic

properties. The supercapacitor was readily stabilized during

reduction. A specific capacitance of 223.6 F g�1was achieved

at 5 mV s�1, while a gravimetric value of 180.8 F g�1 at

0.2 mA cm�2 could be well maintained up to 1000 cycles. The

superior and stable capacitive performance renders ERGO as

a promising material for energy storage.

Acknowledgements

This research was financially supported by a Federal HATCH

grant from College of Agricultural and Life Sciences at the

University of Wisconsin-Madison.

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